Why do power stations have cooling towers?

[einsteinsdog]

This might seem a silly question but the answer eludes me . Why heat water , only to cool it then heat it again etc. In any case the waste heat can be used for community heating as is done on the continent , or industry. We do like waste in this country dont we

[mattrix100]

Water is super heated to steam so it turns the turbins (creating electricity).

So that no new water is need, it is simply cooled (back into water) and re-used. It saves money and the enviroment

[einsteinsdog]

Ok fine I get that bit , but what im saying is that at least 75% of energy input is going up in steam!!!!!

[Buenchico]

If you've ever seen steam coming from a power station then there's something seriously wrong with it! Water vapour yes, steam no! That's not just a pedantic point. Steam (and particularly super-heated steam) carries a lot of heat energy. The water vapour coming from power stations contains far less heat energy and, consequently, the energy losses to the system are far less than you seem to assume.

Of course, you're correct to say that it would be possible to use excess heat from power stations for domestic or industrial purposes but this can only be energy-efficient if the factories or houses are immediately adjacent to the power station. Are you volunteering to live on a housing estate, surrounded by electricity pylons, next to a power station?

Chris

[einsteinsdog]

Ok so why have cooling towers as it takes more energy to heat cold water than warm or hot water. Maybe heating the atmosphere is the idea

[Buenchico]

What goes into the cooling towers (rather than comes out of them) is steam but what is needed at the 'input' end of the system is water. The function of the cooling towers is convert the steam back to water. (So it's steam which is being cooled, rather than water). Water condenses on the inside of the towers and run downs the walls. (The shape of the towers is designed to minimise the loss of water vapour while ensuring that the internal water flows efficiently to the reservoirs at the bottom). The water which arrives in the reservoirs (and is then returned to the system) is still hot. i.e. while the steam has undergone cooling the resultant water has not.

Chris

[mastadon]

cooling towers are only presnt in nuclear power stations. they are used to help regulate fuel rod temperature

[Buenchico]

Mastadon's statement that cooling towers are only found at nuclear power stations will come as something of a shock to the residents of Selby. They've always assumed that the fuel source for the nearby Drax power station (which is surrounded by cooling towers) might just have something to do with the millions of tonnes of coal which are delivered there!

http://www.draxpower.com/

Chris

[einsteinsdog]

Right got it now , basically low pressure steam is no use because it has less energy in it. so it is condensed to water to make it easy to heat again into high pressure steam

[brachiopod]

After the high pressure steam has passed through the turbine-generator unit, it needs to be cooled to reduce the pressure and thus maintain the pressure gradient across the turbine. Cooling the steam greatly reduces the pressure, ensuring more high pressure rushes in to replace it, turning the turbine as it goes.

So, after passing through the turbine, the steam passes through a heat exchanger, where a large volume of cold water from an abundant and nearby source (river, lake or sea) cools the steam rapidly, but only sufficiently to condense it, which reduces the pressure considerably. In doing so, the cold river water (the coolant) is heated up slightly, so to maintain the thermal gradient, (ie cold river water against hot steam pipe), the flow rate of the coolant water is high, and therefore large volumes are required. The two flows (high pressure steam and coolant) are separate systems and do not mix.

The waste coolant water is then returned to its source, (eg river, lake etc). However, before this can be done, it needs to be cooled, since it is a good few degrees above its original abstraction temperature. This is done by percolating it through cooling towers, which are designed to create an updraught of air from their open bases to assist the process.

IT IS THIS 'WASTE' COOLANT WATER THAT IS BEING COOLED IN COOLING TOWERS, NOT THE HIGH PRESSURE STEAM FROM THE TURBINE SYSTEM.